Method of making steel powder



Sept. 8, 1970 R. A. HUSEBY E AL 5 METHOD .DF MAKING STEEL POWDER Filed Jan. 18, 1968 ADD SULPHUR TO MOLTEN STEEL ATOMIZE MOLTEN STEEL TO FORM AGGLOMERATED PARTICLES FIG.] I

ANNEAL PARTICLES BREAK UP CAKE TO AS-ATOMIZED PARTICLE SIZE GOMPRESS AND SINTER PARTICLES TO FORM STEEL ARTICLE INVENTORS ROBERT A. HUSEBY PAUL B. GARNER Attorneys United States Patent l 3,528,081 METHOD OF MAKING STEEL POWDER Robert A. Husehy, Milwaukee, and Paul B. Garner,

Menomonee Falls, Wis., assignors to A. O. Smith Corporation, Milwaukee, Wis a corporation of New York Filed Jan. 18, I968, Ser. No. 698,738 Int. Cl. B221 1/00 U.S. Cl. 75-5 4 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a method of making steel powder having, when compacted and sintered, increased machineability. Sulphur is added to carbon steel in the furnace and/or ladle in an amount of about .08 to .25% by weight of the steel. Subsequently, the molten steel is fed by gravity from a tundish in the form of a downwardly moving stream and sheets of water are impinged against the stream to atomize the molten steel and provide a plurality of chain-like agglomerates of particles contain ing fine, dispersed inclusions of sulphur. The particles are subsequently annealed at a temperature of 1500 F. to 2100 F. in a reducing atmosphere to soften the steel particles, reduce the oxide film and decrease the carbon content. When subsequently compacted and sintered, the resulting metal part has improved machineability due to the sulphur dispersions which are locked within the steel particles.

There are several basic procedures by which metal powder, to be used in powder metallurgy processes, can be prepared. For example, metal powder can be prepared by electrolytic processes, ore reduction processes, or by air or water atomization processes, such as that described in U.S. Pat. 3,325,277. According to the process of that patent, molten steel, having a carbon content less than 1.8%, is fed by gravity from a tundish in the form of a downwardly moving stream. A series of flat sheets of water are impinged against the stream of molten steel at an angle to thereby atomize the molten stream and provide a plurality of chain-like agglomerates of spheroidal particles. Subsequently, the particles are heated or annealed to a temperature of about l500 F. in a reducing atmosphere for a period of time sufiicient to soften the particles and reduce the carbon content to a value less than 005%. Following the annealing treatment, the particles are subjected to hammer milling to break up the cake-like structure formed during the anneal and restore the asatomized particle size.

The steel powder formed according to the method of Pat. 3,325,277, when compacted and sintered, has a high density and superior physical properties.

The steel powder such as that produced by the process of the above patent, is generally used for the production of metal parts having complex contours or shapes, for the powder can be molded to the complex shape, and machining of the part is thereby minimized. In spite of the fact that powder metallurgy processes are designed to eliminate or minimize machining, most powder metallurgy parts nevertheless require some minor machining such as tapping of threads, drilling of holes, undercutting, and the like.

The present invention is directed to an improvement to the process described in Pat. 3,325,277 and is directed to the production of steel powder, which when compacted and sintered, has increased machineability. According to the process of the invention. sulphur is added to the mol ten steel, either in the furnace and/or the laddle. Subsequently, the molten steel containing the sulphur in solution, is fed by gravity from a tundish in the form of a 3,528,081 Patented Sept. 8, 1970 downwardly moving stream. A series of sheets of water are impinged against the stream of molten steel and act to immediately quench and atomize the steel to provide a plurality of clump-like agglomerates of particles. Due to the very rapid quench involved. the sulphur ispresent in the form of fine dispersions throughout the steel particles and large inclusions of sulphur are eliminated.

Subsequently, the particles are annealed at an elevated temperature and the annealing not only acts to soften the particles and reduce the carbon content but also serves to burn off any residual sulphur which may be located on the surface of the steel particles. During the anhealing. the particles tend to bind together to provide a cake-like structure and following the anneal, the cake-like structure is broken up to restore the as-atomized particle size of the agglomerates.

Subsequently, the particles are compressed and sintered under conventional procedures and the resulting metal part has improved machineability due to the presence of the suhphur. However, as the sulphur is locked within the individual steel particles, the particles can be compressed and sintered without any appreciable loss in physical properties.

When machining the finished part, the machined surface is extremely smooth and the chips are short and bright with no color discoloration due to heating.

Other objects and advantages will appear in the course of the following description.

FlG. l is a flow sheet indicating the process of the invention, and

FIG. 2 is a schematic representation of the apparatus employed for atomizing the metal particles.

The molten steel to be used in producing the steel particles can be produced by any of the conventional steel making process, such as open hearth electric furnace, basic oxygen, and the like. The steel contains less than 3.3% by weight, and preferably less than 2%. of total alloying elements and the carbon content is less than l.8% by weight, preferably less than 0.20%, and under most conditions in the range of 0.08 to 12% by weight. In addition, the steel has a manganese content less than 0.60% and generally in the range of 0.15 to 0.40% and preferably in the range of 0.25 to 0.35%.

According to the invention, sulphur is added to the molten steel either in the furnace or in the laddle or in both the furnace and in the ladle. The sulphur is added in an amount so that the atomized powder has a sulphur content in the range of 0.08 to 0.25% by weight and preferably in the range of 0.10 to 0.20%. Normally, the amount. of sulphur added to the molten steel will be slightly in excess of the above range due to the fact that some sulphur will be lost during the steelmaking process.

The sulphur can be added to the molten steel in any desired form. It has been found that sulphur in the form of solid sticks are very satisfactory for the sticks can be completely immersed in the steel without any appreciable burning oil of the sulphur. As an alternate method, the sulphur can be added to the molten steel in the form of substantially pure pyrite, or elemental sulphur can be added as a liquid or as a powder.

The molten steel containing the sulphur is then introduced into tundish l at a temperature of about 3100 F. and the molten steel flows by gravity through the outlet slots or nozzles 2 in the form of a series of streams 3. The process by which the steel is atomized can be similar to that described in Pat. 3,325,277 of Robert A. Huseby, entitled Method of Making Metal Powder.

To atomize the stream to molten metal, water in the form of thin sheets or curtains 4 is directed against the stream 3 at an angle greater than 5 with respect to the axis of the stream and generally at an angle of 15 to from the vertical. The temperature of the water employed in the atomization process is not critical and is generally less than 160 F.

The water employed during the atomization is under substantial pressure above 500 p.s.i. and for most operations above 1.000 p.s.i.

The water is directed toward the stream of molten metal in the form of a thin sheet having a thickness less than 0.075 inch and preferably less than 0.05 inch at the point of discharge from the nozzle. The nozzle is designed so that the water sheets do not flare out to any appreciable extent but maintain the thickness when impinging against the molten steel stream.

The thin sheets of water act to atomize or particlize the steel and produce chain-like agglomerates of generally spheroidal particles. The irregular shape of the agglomerated particles results in a higher green strength for articles subsequently made from the steel powder.

The sulphur is in solution in the molten steel and due to the rapid quenching brought about by the atomization,

the sulphur is trapped or locked in the individual particles in the form of fine dispersions. The rapid quench prevents the dispersions of sulphur from accumulating in the form of large inclusions and it is believed that the fine dispersions which are locked within the individual particles is responsible for bringing about the improved machineability of the subsequently sintered parts without any appreciable loss in other physical properties.

Following the atomization, the steel powder is subjected to an annealing treatment which serves to soften the particles, reduce the oxide film and substantially decrease the carbon content. In addition, any sulphur dispersions which are exposed on the surface of the particles will generally be burned off during the annealing treat;-

ment, so the annealed particles are free of external 3 sulphur.

During the annealing treatment, the powder is heated to a temperature in the range of 1500 F. to 2100 F. and preferably 1650 F. to 1800 F. in a reducing atmosphere such as disassociated ammonia, hydrogen, or other conventional decarburizing reducing gases. The annealing treatment serves to soften the steel particles as well as reducing the carbon content to a value to below 0.05% and generally to a value in the range of 0.001% to 0.020%. To obtain the optimum physical properties, the powder should be held at the annealing temperature for a period of at least 1.5 hours and preferably 2 hours.

During the anneal, the particles tend to cake together and the cake-like structure is broken up after the anneal by a hammermill process. The hammermilling breaks up the sintered cake, while not breaking up the regular agglomerized nature of the particles, and restores the original atomized particle size. It is important to control the dewpoint in the annealing operation to prevent excessive caking of the particles. Preferably the dewpoint should be above 50 F. and below 80 F. in the last /3 of the hot zone of the furnace.

The resulting steel powder can be used to form any desired machine part or combination of parts by conventional powder metallurgy processes. For example, a conventional lubricant, such as zinc stearate and additional carbon if desired, can be blended with the steel powder by suitable blending equipment. The blended powder is then compacted into the desired shape by a compaction pressure generally above tons per square inch and preferably about tons per square inch.

The compacted powder is then sintered in a reducing atmosphere at a temperature in the range of 2,000 to 2,300 P. for a period of 10 minutes to 1 hour, depending on the composition and the final density desired.

The addition of the sulphur substantially improves the machineability of the sintered part. The sulphur is believed to be present in the form of fine manganese sulphide dispersions which interrupt the steel matrix and cause the chips, during machining to break immediately ahead of the tool, resulting in a lower coefficient of friction between the tool and the work. The chips formed during machining are generally short and bright with no discoloration due to excessive heating. Furthermore, the machined surface is considerably smoother than the machined surface of a similar part without the addition of sulphur. For example, typical surface roughness measurements after identical machining operations show an arithmetic average surface roughness of 100 microinches on sintered metal prepared in accordance with the invention as compared to an average surface roughness of 250 microinches on identically processed sintered metal but without the addition of sulphur.

In prior art processes, sulphur has been mixed with the steel particles prior to compacting and sintering in an attempt to increase the machineability of the sintered part. During the sintering, the sulphur forms a film or a coating on each individual steel particle and this sulphur film seriously reduces the mechanical or physical properties of the sintered part. In addition, the surface sulphur can cause serious damage to alloy mesh belts and furnace mufilers during sintering. By addition of sulphur to the ladle or furnace, as in the present invention, the sulphur is locked within the particles so that there is no layer or film of sulphur on the individual particles which can detract from the physical properties of the sintered part. While the sulphur provides some embrittlement for the steel, the fact that the sulphur is locked within the particles, enables the sintered part to have a substantially higher green density and green strength than sintered parts produced by conventional techniques in which the sulphur is merely mixed with the atomized particles.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

We claim:

1. A steel powder composition to be used in powder metallurgy processes, comprising a plurality of steel particles with said steel having a carbon content less than 0.05% by weight and a manganese content less than 0.60% by weight, and a plurality of fine dispersions of sulphur distributed throughout the interior of said particles, said sulphur being present in the range of 0.08 to 0.25% by weight of the steel, said sulphur dispersions acting to improve the machineability of a metal part subsequently formed by compacting and sintering said particles.

2. The steel powder composition of claim 1, wherein the sulphur is present in the range of 0.10 to 0.20% by weight of the steel.

3. The steel powder composition of claim 1, wherein said steel particles are in form of agglomerates of spheroids.

4. The steel powder composition of claim 1, wherein the outer surfaces of said particles are substantially free of sulphur.

References Cited UNITED STATES PATENTS 1,739,052 12/1929 White 0.5 2,942,334 6/1960 Blue 75-0.5 3,132,021 5/1964 Koehring 75-05 3,141,760 7/1964 Finke et a]. 75-0.5 3,325,277 6/1967 Huseby 750.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner 

